What are meta tags?

The Meta tag in html is not a required tag when you're creating your web pages; many pages don't use the tag at all, and I must confess that I've not used it on my home page, although I put it into this page by way of demonstration. To put it briefly, the meta tag is used by search engines to allow them to more accurately list your site in their indexes. Sometimes.
If you simply produce your home page and register the URL with a search engine, or a number of search engines, their spider programmes will (eventually) toddle along to your site to index it. Now, each of the search engines does this slightly differently. Altavista for example will grab everything in your document and index it, but will only show the first 250 characters in its description. Consequently, if your site included say, 'Thanks to:....' right at the beginning, this is what Altavista would show in its description, and it wouldn't give the viewer any idea of what your site actually covered. Of course, not all search engines work this way; I'd suggest you ferret around a little bit to see exactly how the popular engines work, and certainly the major two or three such as Altavista, Lycos, Yahoo, Infoseek and Excite. It therefore makes sense to ensure that your opening paragraph is carefully written to ensure it accurately reflects what your site covers.
However, you may be able to exert a certain amount of control over how your site is indexed by the use of the meta tag. (I should however point out that not all search engines will use this tag - Altavista does, but Excite doesn't, for example. Its not a total cure-all therefore, but you won't miss out by putting the tag in, and it may well work well in some cases.